1. Field of the Invention
The present invention relates to the technical field of optical networks. More particularly, the present invention relates to relay nodes used in the optical networks of the Wavelength Division Multiplexing (WDM) scheme.
2. Description of the Related Art
As demands for communications such as the Internet, broadband services and the like increase in recent years, necessity for increasing distance, speed and capacity of the communications is increasing more and more. In the WDM scheme, lights of various wavelengths are multiplexed and transmitted so that high-speed and large-capacity can be realized. A relay node used in the conventional WDM optical network performs routing (transferring) by converting an optical signal into an electrical signal, then, converting the electrical signal into the optical signal again. However, the converting processing between the optical signal and the electrical signal increases the overhead in routing, so that it causes complexity of processing and causes transmission delay. The following patent document 1 discloses a technique for performing path setting such that the transmission distance becomes long as much as possible without optical/electrical/optical (O/E/O) conversion.
In recent years, the GMPLS (Generalized Multi Protocol Label Switching) protocol is widely noticed as a protocol for automatically setting a path (to be also referred to as “routing route”, “connection”, etc.) by recognizing wavelengths of various colors of lights as labels as it is. In the GMPLS, the routing route is determined based on the wavelength of the optical signal, and data is transmitted as the optical signal, so that the problem due to the electrical signal conversion can be largely decreased.
[Patent document 1] International Publication No. 2005/032076 pamphlet
For every optical signal of every wavelength provided in the system, if the relay node forming the optical network can perform signal adding from low-speed side ports to high-speed side ports or can perform signal dropping from the high-speed side ports to the low-speed side ports, desired connections may be constructed quickly and automatically using the GMPLS protocol. However, the high-performance relay node that can use all of the various wavelengths without limitation is very expensive and such relay node is not so widely used actually. Rather, there are many inexpensive apparatuses having limitations in wavelengths and in input/output ports that can perform add/drop in an optical fiber. By the way, the relay node is a node that performs both or one of adding an optical signal into the optical fiber of the WDM optical network and dropping the optical signal from the optical fiber, and the relay node may be called an Add/Drop Module (ADM) or Add/Drop Multiplexing Module or the like.
In the relay node shown in FIG. 1, any of the four kinds of the wavelengths λ1-λ4 can be added to or dropped from the optical fiber. In the example shown in the figure, the low-speed side port #2 is different from the low-speed side port #1, and the low-speed side port #2 cannot add an optical signal to the WDM port #10 side and cannot drop an optical signal from the WDM port #10. But, since there is no limitation for wavelengths to be dropped or added in this relay node, this relay node belongs to the before-mentioned expensive relay node, and is also called DOADM (Dynamic Optical ADM). On the other hand, in the low-speed side port #1 of the relay node shown in FIG. 2, only an optical signal of a first wavelength λ1 can be dropped or added. In the low-speed side port #2, only an optical signal of a fourth wavelength λ4 can be dropped or added. Optical signals of second and third wavelengths λ2, λ3 cannot be dropped or added. This relay node belongs to the above-mentioned inexpensive apparatus, and may be called ROADM (Reconfigurable Optical ADM).
When there is a limitation for the wavelength that can be added/dropped in the low-speed side port in the relay node, connections cannot be properly established only by using the connection setting method of the GMPLS protocol. Therefore, it may be considered to establish a connection manually by checking constraints of individual relay nodes at each time (especially, kinds of usable low-speed side ports, and information of wavelengths that can be added/dropped). However, such method is practically very inconvenient. Or, it may be considered to provide a central management station for managing constraints of all relay nodes used in the optical network collectively and providing instructions when setting the connection. However, such management workload is extremely heavy so that it is feared that it becomes difficult to set the connection quickly as necessary.